Jeff Cooper, in The Art of the Rifle, stated that the seated position is the most useful for hunters. Military shooters use it less because it’s neither as low as prone nor fast like squatting or kneeling.
The sitting position is moderate to high stability, depending on your body mechanics. You will get slightly lower than kneeling or squatting but stay above most brush and grass. Kyle Lamb, in Green Eyes Black Rifles, points out that sitting is also convenient because it can be held for a long time compared to some of the others.
Properly done with a sling, I’ve found that you get almost as much stability as you do in prone. In fact, some of my best groups came from the seated position. Let’s look a little closer.